The present invention relates to the method of forming a fluorescent screen of a color picture tube by way of so called dry process wherein powdery phosphor is applied to the face panel.
The fluorescent screen of a color picture tube is formed of a layer of phosphor dots or stripes of three primary colors arranged precisely at predtermined positions, so that picture elements of the screen can luminesce at three primary colors. A widely-used method of forming the fluorescent screen is that a photosensitive material is applied to the inner surface of the face panel to form a film, portion of the film where the phosphor layer is going to be formed is exposed to light through a shadow mask, and the phosphor layer is formed by use of photo-reaction which takes place at the exposed portion. This method falls into wet process which is used in general and dry process which has been developed recently. One may refer to U.S. Pat. application Ser. No. 895,372, filed Apr. 11, 1978, now U.S. Pat. No. 4,273,842, disclosing a dry process.
According to such a dry process, a photosensitive resin mainly containing diazonium salt is applied to the inner surface of a face panel to form a film. Then portion of the film where a phosphor layer is going to be formed is exposed to light so that zinc chloride having the moisture absorptive characteristic is produced in the exposed portion of the film by photo-reaction. The exposed portion becomes sticky when it absorbs moisture in the air. Next, phosphor powder having a desired luminous color is sprayed onto the inner surface of the face panel, so that the phosphor powder adheres only to the sticky portion of the film. This process is carried out for each of green, blue and red phosphors in the case of a color picture tube. The phosphor layers formed in this manner has the water-soluble characteristic and cannot sustain filming which will be carried out subsequently as a post processing, and therefore a fixing process is carried out using ammonia gas so that the phosphor layers are made water-insoluble. However, the additional fixing process is not profitable for productivity, and there has been proposed a method for solving this problem wherein a photosensitive binder such as ammonium bichromate is beforehand added to the photosensitive resin. (See U.S patent application Ser. No. 64,739, now U.S. Pat. No. 4,247,612).
However, the photosensitive resin mixed with the photosensitive binder is so sensitive to dark reaction that the nature of the photosensitive resin changes by dark reaction. Therefore, the mixture cannot be stocked and must be used up at once, resulting in a poor productivity.
The "dark reaction" herein means that the binder is reacted with light of wavelengths other than the characteristic wavelength of the binder, or with heat.